W140s have soundproofing insulation glued to the under side of the roof inside the sunroof well. This stuff is famous for falling down and screwing-up the sunroof slider mechanism. It looks like paper-backed linoleum tile and is glued to the roof with a very sticky tar-like substance. The problem is that in warm climates this tar liquefies in the hot sun and allows the insulation to fall down. I had this problem on both my cars.
It's relatively easy to fix if you can get the sunroof enough to slide something in there and knock the insulation out of the way. I actually went so far as to remove the sliding liner under the roof. I was then able to reach in with my hands and various tools and completely pull the stuff out.
Here's a link to a photo DIY I posted on benzworld. One thing to note is that this procedure was done on a car with a glass sunroof. This may or may not work for a metal one.
http://www.benzworld.org/forums/w140-s-class/1448445-photo-diy-removing-sunroof-liner.html
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"It's not about how fast you can go, but how well you can go fast."
Bob in Richmond
'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles
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